(1) Field of the Invention
This invention is concerned with lockstitch sewing machines, especially, but not exclusively, lockstitch sewing machines for use in the shoe industry where, for example, outsoles of thick, tough material, e.g. leather, are stitched to welts in the production of welted shoes.
(2) Prior Art
One such machine is described in UK Patent Specification No. 410463 and comprises a worktable and a presser foot arranged at an operating locality of the machine for clamping a workpiece therebetween and feeding it step-by-step through the operating locality, a mounting for a curved hooked needle and a further mounting for a curved awl arranged in a common vertical plane and at opposite sides of the worktable, at the operating locality of the machine, and mounted for pivotal movement about a common axis, means for looping thread over the needle hook, after the latter has penetrated a workpiece being stitched and while it is projecting therethrough, retraction of the needle then causing the thread to be drawn through the workpiece, means, including a rotatable shuttle, by which a lockstitch can be formed with that thread and a further thread, and means for thereafter drawing the lockstitch thus formed into the body of the workpiece. In such machine, furthermore, the needle mounting and the awl mounting are caused to pivot each by a crank mechanism.
In the machine described in the aforementioned patent specification, the general arrangement of mechanisms is very complex, and in particular the crank mechanism for the needle mounting and awl mounting movements are driven compositely by two separate, but interconnected, crank shafts one of which runs at a substantially higher speed than the other, the arrangement being such that the output of the crank mechanisms, which are mounted on the one crank shaft, are modified by the action of further mechanisms associated with the other of the crank shafts. In operation, this arrangement of two crank shafts has proved highly successful in a high speed machine manufactured and supplied by the applicant company; by "high speed machine" in this context is to be understood a machine capable of operating at over 1000 stitches per minute. However, the arrangement of two crank shafts is clearly expensive in terms of material costs and machining costs, and furthermore, because of the complexity of the arrangement, a great deal of assembly skill is required in building the machines. In addition, not only is a good deal of space required within the machine frame for accommodating this double crank shaft arrangement, but further a relatively large number of pivots is involved, with the consequent problems of tolerances and a general "springiness" in the mechanisms. What is more, in the particular arrangement described in the aforementioned patent specification, an acceleration analysis indicates that the acceleration of various of the integers, notably the needle mounting and the awl mounting, are subject to sharp peaks, which can result in excessive wear and increased noise levels.
It is therefore the object of the present invention to provide an improved lockstitch sewing machine which can operate at a high speed, but the mechanism of which are significantly simplified and rendered more compact, and the peak acceleration of the various components are "rounded" so as to enhance wear and reduce noise levels.